1. Field of the Invention
The present invention relates to an adhesive composition that is halogen-free, and yields a cured product, upon curing, that exhibits excellent flame retardancy and anti-migration properties, and also relates to an adhesive sheet, a coverlay film, and a flexible copper-clad laminate that use such a composition.
2. Description of the Prior Art
In recent years, developments within the electronics field have been remarkable, and in particular, communication and consumer electronic devices have seen considerable progress in terms of device miniaturization, weight reduction, and increased component density. Demand for this type of improved performance continues to grow. In response to these demands, flexible printed wiring boards exhibit favorable flexibility and are resistant to repeated bending, meaning they can be packaged three dimensionally at a high density within a confined space. Accordingly, they are being used more and more widely as composite components that include functions such as the wiring, cabling or connectors to electronic equipment.
A flexible printed wiring board is produced by forming a circuit on a flexible printed wiring board substrate using normal methods, and then depending on the intended use of the wiring board, bonding a coverlay film to the board to protect the circuit. The flexible printed wiring board substrate used in the flexible printed wiring board is a laminate prepared by using an adhesive to bond a metal foil to an electrically insulating film that exhibits a high level of heat resistance as well as excellent electrical and mechanical properties. The properties required for the flexible printed wiring board substrate include favorable adhesion durability, as well as favorable levels of heat resistance, flexibility, foldability, anti-migration properties, and flame retardancy. Furthermore, an adhesive sheet refers either to a sheet that is used for laminating together two or more single-sided copper foil or double-sided copper foil flexible printed wiring boards, thereby forming a multilayer structure, or to a sheet that is used for bonding a reinforcing sheet to a flexible printed wiring board. The properties required for such adhesive sheets include favorable adhesive strength, heat resistance, and anti-migration properties.
Against the background of recent environmental problems, there is a growing trend to limit the use of halogen compounds in components mounted within electronic equipment, meaning the use of bromine compounds, which have conventionally been widely used to impart flame retardancy to the materials used for flexible printed wiring board substrates, is becoming increasingly difficult.
As a result of the environmental problems mentioned above, recently, a technique has been adopted in which a phosphorus-based flame retardant compound is added to the adhesive instead of a bromine compound in order to achieve the required flame retardancy. For example, resin compositions have been proposed that comprise an epoxy resin, a phosphate ester compound, a phenol-based curing agent, and an NBR rubber as the primary components (see patent reference 1 and patent reference 2). However, phosphate esters exhibit poor resistance to moisture and heat, meaning that under conditions of high temperature and high humidity, the phosphate ester generates ionic components via hydrolysis, resulting in unsatisfactory levels of anti-migration properties, peel properties and solvent resistance in the resulting substrate. Furthermore, compositions comprising a phosphazene compound, a polyepoxy compound, a curing agent, a curing accelerator, a synthetic rubber and an inorganic filler have also been proposed (see patent reference 3 and patent reference 4), but the peel properties, solvent resistance and solder heat resistance of the resulting substrates are not entirely satisfactory.
Moreover, research by the inventors of the present invention has revealed that even if a composition exhibits excellent anti-migration properties within widely used conventional single layer structures, if a plurality of flexible printed wiring board substrates are laminated together to increase the circuit density, or if other heat loading (heat history) is applied repeatedly, then the resulting anti-migration properties may be inadequate. Accordingly, further improvements in the anti-migration properties are required.
[Patent Reference 1] JP 2001-339131A
[Patent Reference 2] JP 2001-339132A
[Patent Reference 3] JP 2001-19930A
[Patent Reference 4] JP 2002-60720A